Planning moves forward for new collider to follow up on LHC's discoveries.

The Large Hadron Collider (LHC) is currently undergoing upgrades that will allow it to finally reach its intended top energy of 14TeV. When it comes back online, researchers will use it to probe the properties of the Higgs boson it discovered and to continue the search for particles beyond those described by the Standard Model. But no matter how many Higgs particles pop out of the machine, there's a limit to how much we can discover there.

That's because the hadrons it uses create messy collisions that are hard to characterize. The solution to this is to switch to leptons, a class of particles that includes the familiar electron. Leptons present their own challenges but allow for clean collisions at precise energies, allowing the machine to produce little beyond the intended particles. So now, the international physics community is putting agreements in place that will see a new lepton collider start construction before the decade is out, most likely in Japan.

Hadrons vs. leptons

Hadrons like the proton are composed of a mixture of quarks, gluons, and virtual particles. Their heavy mass makes them easy to accelerate. When fast-moving particles move around curves, some of their energy is lost as radiation; the proportion of lost energy goes down as the particle's mass goes up. So when lightweight electrons were sent through the curved tunnels that now house the LHC, the maximum energy they could reach was just over 100GeV. In contrast, protons can go through those same curves at 7TeV, meaning the collisions have significantly more energy.

The problem with hadrons comes from the fact that they're made up of a bunch of individual particles, and there's no way of controlling how the particles are oriented when the collision takes place. Even when two protons smash head-on, the quarks inside them may end up colliding off-center. As a result, only a small fraction of the total energy ends up being available to create new particles. Since there's no way of predicting how much, reconstructing what happened during a collision gets rather complicated, especially since each collision is really multiple smaller collisions. Imagine trying to reconstruct what happened when two trucks used to ship cars collide when you don't know which cars started on what truck.

Leptons, in contrast, are fundamental particles; there's nothing inside them, so when they collide, all the energy gets put into a single, simple collision. This means you can see things like the Higgs without accelerating the electrons to anywhere near the energies used at the LHC. It's also relatively easy to collide electrons with their antimatter cousins, the positrons, which eliminates the particles themselves in the process. The end result is a very clean collision and, if it's at the appropriate energy (say, near the mass of a heavy particle), a strong likelihood of producing the particle you're interested in.

Since leptons don't like running in circles, the solution is to build a linear collider. If everything moves in a straight line, it's possible to get electrons up to high energies and keep them there. The problem is that a straight line means long tunnels and infrastructure spread along them. All of which means a large expense.

ILC vs. CLiC

Even while the LHC was still being built, scientists started planning for a next-generation linear collider under the assumption that the LHC would spot the Higgs and physicists would want to study its properties in detail. Two competing camps were formed, one focused at Fermilab, the other at CERN.

The CERN group called its effort the Compact Linear Collider, or CLiC. To keep costs under control, CLiC would accelerate leptons within a shorter distance by transferring energy from one beam of electrons to the beams of electrons and positrons used for collisions. The result is a shorter (or, as the name implies, more compact) collider. The downside is that the technology remains unproven, so we don't know whether it would actually work in practice.

That dilemma left the focus on the International Linear Collider, which uses extensions of existing technology on a larger scale—a much larger scale. The initial plan for the ILC calls for the hardware to be housed in a tunnel 30km long; to increase the energy later, another 20km may be added. Initial plans were to build it at Fermilab, although the size meant that the tunnel would extend out under some of the neighboring towns. But the US Congress cut the funding for it a few years back, leaving the project in a bit of a limbo. Two factors have changed that: the actual discovery of the Higgs, which gives the collider its purpose, and the Great East Japan Earthquake of 2011, which freed up money from the Japanese government as part of the country's recovery efforts.

(It's worth noting that Japan hasn't been designated the official site yet, but that is an offer that will be hard to refuse. Also worth noting is that the ILC and CLiC teams are partly competitors and partly collaborators.)

Last week, the ILC team published its technical design report, which lays out how the accelerator will function and what it might explore.

The design calls for two storage rings, one each for the electrons and positrons. Electrons are relatively easy to come by, but the positrons will be made on-site. Basically, the electron beam will be sent on a curved path and lose energy via the production of high-energy photons. These photons will be converted to electron/positron pairs; the electrons will be discarded, and the positrons will be sent into the storage ring. From the ring, the electrons and positrons will be sent 15km in opposite directions, sent around a curve and then fed into two linear accelerators that are 11km long each and pointed roughly at each other.

If future higher-energy extensions go forward, they would basically involve extending the tunnels and shifting the point where the electrons go around a bend further out. More accelerator hardware would be attached to the backend of the existing linear accelerator, lengthening it so that the electrons spend more time being accelerated and come out at a higher energy.

There's a single beam-collision point at the center of the complex, which means only a single detector can take data from collisions. To provide the reassurance of reproducibility, there will be two detectors on rails, allowing one to be slid out and a second to be slid in to replace it. If there's enough shielding, it should be possible to perform maintenance on one while the second is operating.

Since the amount of acceleration can be tuned, the initial range of the accelerator runs from about 200GeV to 500GeV, enough to explore the production of the Higgs with a Z boson and see how the two interact (given that the Z is a relatively massive particle, the answer is expected to be "extensively"). At the high end, a pair of Higgs particles can be produced with a Z, allowing us to determine how the particle interacts with itself. Combinations of the Higgs and two top quarks are also possible (the top is the most massive particle we know about).

Extending the energy up to 1TeV would get us more elaborate combinations, but it's hard to see that much money being spent unless the LHC produces some evidence for supersymmetry, additional Higgs particles, or dark matter particles (or some combination of the above).

The LHC has plenty of time to get there, as a final site for the ILC hasn't even been chosen yet. It will take a few years to finalize the design specs and then roughly a decade to construct it, so operation probably won't start until the late 2020s. By that time, the LHC will probably have gone through a second round of upgrades and should have thoroughly explored all the particles accessible to the energies it can reach.

Promoted Comments

So they built a $9 billion dollar machine that spans multiple countries which after only 5 years is obsolete because the test environment is too sloppy after the discovery ? That sucks - giant paper-weight.

This was always the plan. You need a hadron machine to discovery a heavy particle and then you need to build a lepton machine to really study it in complete detail. An example is the SPS which discovered the W and Z bosons and LEP which really studied them in detail. There was always the plan to build a linear collider to study whatever the LHC found.

Why do we need a hadron machine to discover things? Well as discussed in the particle, hadrons are comprised of quarks and gluons which each carry a fraction of the hadron's momentum. Therefore the interaction energy can take any value up to the beam energy (there is a defined distribution of these energies but lets not go there). This is great for discovery because you just give the protons enough juice (and as they are heavier, its easier to do this) and then you can probe a whole spectrum of masses, no need to know in advance which one it is. A linear collider on the other hand, you have to specify the collision energy and this is related to the mass of the particles you want to produce. If you dont know this mass, well you're kinda stuck But conversely once you know the mass from the hadron collider, you can tune your collision energy to produce lots of that particle.

Also, the LHC is not obsolete, it still has an extremely rich physics program ahead of it. It is true the ILC will do better measuring the Higgs properties (with the exception of Higgs self couplings), but there are many other things the LHC is looking for such as extra dimensions, dark matter, new gauge bosons and susy.

Summary: The "sloppiness" of the environment is sadly a necessary feature for discovery and the LHC still has many stories left to tell, its going to be an exciting 2015

54 Reader Comments

What determines which kind of particle is produced in the collision? The article says it's likely if it's approximately the collision energy, but how is that known?

The theory holds that certain energies(masses) correspond with certain particle collisions based on the number and type of subatomic particles. The only way to find out for sure is to build these REALLY expensive devices and test (multiple times as quantum effects can only be mapped statistically).

The theory holds that certain energies(masses) correspond with certain particle collisions based on the number and type of subatomic particles. The only way to find out for sure is to build these REALLY expensive devices and test (multiple times as quantum effects can only be mapped statistically).

There's no field theory that gives numerical answers?

The numerical answer is in the form of a probability distribution -- if A and B collide with energy C, then there's a Y% chance of getting D and Z% chance of E. That's the only kind of answer you ever get in the quantum world.

So the only way to check experiment against the theoretical prediction is to collect a ton of data and see if over time the data looks like the predicted statistical distribution. A single collision will tell you essentially nothing, like trying to tell if a coin is fair by flipping it once.

Build both. That way you can turn money into Higgs particles twice as fast.

"First rule in government spending: why build one when you can have two at twice the price? Only, this one can be kept secret. Controlled by Americans, built by the Japanese subcontractors." - S.R. Hadden

Write better software for untangling hadron collisions. With the Sparks initiative bringing many distributed CPU's to bear on the problem would seem to be smart until the linear collider could be built.

I'm a big fan of pure science. But at what point are the billions of dollars and untold man-hours of some of our best and brightest better served in other areas of science?

"...The better question is why are we spending billions on war when we should be working together on everything else."

Because science is best pursued by a free (not oppressed people). And there are those who try to oppress others. Both defense and the pursuit of science are important, in fact go hand in hand sometimes.

The better question is why are we spending billions on war when we should be working together on everything else.

Apparently some are more focused on their deities than on our world. The "What *I* believe is true and what *you* believe is false" and the "fuck *them*, *I* am more important" mentality is always going to be a source of problems, and it spans a wide array of situations from the kindergarten fights to the conflicts in the middle east. It's in our nature, sadly, and it won't be going away for a good long time, but you already knew that.

Sometimes I feel like we have too much redtape to finish with conflict these days. It was a lot easier centuries ago. Piss off the wrong guy and be prepared to die. These days they just bully you and push you around, so the problem never goes away.

The result is a shorter (or, as the name implies, more compact) collider. The downside is that the technology remains unproven, so we don't know whether it would actually work in practice.

To be fair, the technology is always unproven. The SSC would have had coils with record magnetic field strengths (which they got problems with), the LHC had coils with record curvature (which they got problems with) and record data crunching (which they managed well), et cetera.

I don't think CLiC would be such a stretch in comparison. But it is correct that they aren't extrapolating coils and networks, they are doing new stuff that would need proof of principle demos.

I'm a big fan of pure science. But at what point are the billions of dollars and untold man-hours of some of our best and brightest better served in other areas of science?

I don't like questions that we don't know how to test. If we knew, we could plan science and technology.

We know only two things:

1. Science is among the best ROI areas there are.

I.e. every dime not invested in science is a relative loss.

2. We can never predict exactly which science results will be the recuperating ones.

Foe example, I am responding by www technology, which CERN once invented and premiered to manage their experiments. No reasonable amount of money spent on accelerators will overtake the economic and societal return that gave.

I'm a big fan of pure criticism. But at what point are the billions of cuss word and untold man-hours of some of our best and brightest better served in other areas of science than once again go over what has been scrutinized so many times?

I'm a big fan of pure science. But at what point are the billions of dollars and untold man-hours of some of our best and brightest better served in other areas of science?

I don't like questions that we don't know how to test. If we knew, we could plan science and technology.

We know only two things:

1. Science is among the best ROI areas there are.

I.e. every dime not invested in science is a relative loss.

2. We can never predict exactly which science results will be the recuperating ones.

Foe example, I am responding by www technology, which CERN once invented and premiered to manage their experiments. No reasonable amount of money spent on accelerators will overtake the economic and societal return that gave.

I'm a big fan of pure criticism. But at what point are the billions of cuss word and untold man-hours of some of our best and brightest better served in other areas of science than once again go over what has been scrutinized so many times?

And without the space program we wouldn't have Tang!

(I know it's not true that Tang is a space race spinoff. I just thought it'd be funny to say.)